The potential effects of exposure to naturally occurring radon is currently of interest to The DOE and the general public as well. Although a large data base on the effects of gamma irradiation has been accumulated, information on the effects of radon-emitting alpha particles on biological systems is rather sparse. We have initiated a study to elucidate biochemical and cell cycle perturbation effects of alpha particles on human diploid fibroblast cells. Particular attention is being focused on the involvement of phase-specific checkpoints in both G1 and G2 phases of the cell cycle. The levels of p53 and Waf1 are increased in cells treated with alpha particles and perturbations in the cell cycle lead to an accumulation of cells in both G1 and G2, while cells in S phase traverse at slightly lower rates than control cells. In most recent studies, we have also noted a DNA-PK independent increase of p53 levels in irradiated cells.